“A merry heart doeth good [like] a medicine: but a broken spirit drieth the bones.” Proverbs 17:22.
Let’s start our tour of the body’s organ systems with the skeletal system. Maybe you did not think of your bones as organs, yet they are. When thinking of bones, perhaps you imagine a bleached and dried bone your dog chews, or perhaps a skeleton dangling from a hook at school. While these objects may depict a bone in size and shape, they do not accurately depict a living bone.
Yes, bone is very much alive and dynamic, teeming with active cells, a cement-like matrix holding them together, and the blood vessels that circulate blood through them. Just like any other cell, bone cells need blood to supply them with nutrients and to whisk away waste products. If bones were not alive, they would not heal when they broke. If you have ever fractured a bone, you know how they can hurt. That is because nerves cover their surface (called the periosteum) and make them sensitive to pain.
Our bones change throughout our lives as bone formation and bone destruction occur simultaneously. In our childhood and youth, bone formation occurs more rapidly than bone destruction. After about age 35, bone destruction occurs more rapidly than bone formation. In the elderly, and in some disease processes of the skeleton, bone destruction increases, making bones thin and brittle. Vertebrae may collapse, and height decreases, which is why we may become shorter over time.
Unlike dead bones, living bones are slightly flexible. Many of them are hollow, which makes them lighter and stronger, much like the steelwork in a skyscraper. We are born with over 300 soft bones, but as we get older, many of them fuse together and harden, leaving the adult with 206 bones. The largest bone is the femur in the thigh; the smallest are the auditory ossicles in the ear—you may know them as the hammer, anvil, and stirrup.
Males and females have the same number of bones (including the same number of ribs—24). Otherwise, there are a few minor differences between the male and female skeletons. The bones of the male are generally larger and heavier than the bones of the female. Also, the female pelvic cavity is wider to accommodate childbirth.
What do bones do? One obvious function of bones is to work with muscles to provide movement. You may also realize that bones provide a protective function. Some of our most vital organs are surrounded by bone, such as the brain, the spinal cord, the heart, and the lungs.
One lesser-known function of the skeletal system is that it makes blood. As previously noted, certain bones are hollow. Inside hollow bones are yellow marrow and red marrow. Long bones contain yellow marrow in their hollow central cavities and red marrow in their joint ends. Red marrow can also be found in the cavities of the flat bones, the short bones, and the ribs. Red marrow is where white blood cells and red blood cells grow. The yellow marrow is a storehouse for lipids and minerals, which can quickly be made available for the needs of other parts of the body.
Ligaments connect bones to each other. Articulations, or joints, are where bones meet. Some joints are stationary, such as most of the joints in the skull. It is the use of our muscles, pulling on part of the bones, that causes the movement of a joint.
Calcium phosphate accounts for approximately two-thirds of bone weight. Calcium phosphate crystals are very hard but quite brittle. They can take heavy compression forces, but can shatter when subjected to twisting, bending, or sudden impact.
About one-third of the weight of bone is made up of collagen fibers, which are stronger than steel and flexible, easily tolerating bending and flexing. Collagen’s weak point is that it doesn’t stand up to compression. Collagen fibers make up a framework in which the calcium crystals are locked, forming a protein-crystal complex that blends the strong properties of both substances. Thus bone is as strong as steel-reinforced concrete, but far superior because it is self-repairing. Isn’t God’s design marvelous?
Of what are bones made? Bone cells and other kinds of cells account for two percent of the mass. There are four kinds of bone cells: osteoprogenitor cells, osteocytes, osteoblasts, and osteoclasts. Osteocytes, the most populous type, cannot divide. They help recycle calcium and help in the repair of damaged bone. Osteoprogenitor cells are a type of stem cell that basically grows up to be osteoblasts. Osteoblasts are responsible for producing new bone matrix; so they build up bone. Osteoclasts, huge cells with up to 50 nuclei, remove bone matrix; so they tear down bone. You are probably wondering why we would have cells that tear down bone. As previously stated, bone is a storage complex for minerals. When another part of the body needs a mineral—such as calcium, which also regulates the heart rate and is a part of the blood clotting process—osteoclasts break down the bone matrix to release the mineral, and it is transported via the circulatory system to the area in need. This breakdown happens continually, day by day. If this process were not balanced, our bones would soon fall apart. Here is where the osteoblasts come in, building the bone matrix back up—continually, day by day.
The systems of the body do not work completely independent of one another, but are exquisitely inter-linked (see Table on page 37). Ellen White advised: “The muscles are dependent on the brain and nerves for the power of action. The mind wills to move the limbs. To keep this machinery in working order, it is essential that brain, bone, and muscle be brought into action. The exercise of the muscles quickens the circulation of the blood. How important then that parents understand the philosophy of the healthful action of brain, bone, muscles, and nerves and how needful that they educate their children in this line.” Manuscript Releases, vol. 5, 222.
There are several hormones that play a part in the regulation of bone growth. Calcitriol, made in the kidneys, promotes calcium absorption in the digestive tract. Growth hormone, made in the pituitary gland, and thyroxine, made in the thyroid gland, stimulate osteoblast activity. Estrogen, testosterone, and the parathyroid hormone, made in the ovaries, testes, and parathyroid glands, respectively, stimulate osteoclast activity. Calcitonin, made in the thyroid gland, inhibits osteoclast activity.
Diseases of the bone can be caused by impaired function of the glands. For instance, gigantism results from an overproduction of growth hormone before puberty. This is the disease that the late Robert Pershing Wadlow had, becoming the tallest living person until his death in 1940. His weight was 490 pounds; his height was 8 feet 11 inches (he was 6 feet 5 inches tall when he was ten years old). The opposite problem, pituitary dwarfism, is caused by inadequate amounts of growth hormone. Fortunately these diseases are now more easily treatable than they once were.
We have the opportunity to see giants again in the resurrection; their height not caused by disease. How tall will we grow in heaven? I’d like to find out, wouldn’t you?
“Be not wise in thine own eyes: fear the Lord, and depart from evil. It shall be health to thy navel, and marrow to thy bones.” Proverbs 3:7, 8.